Different from LP unicast, the destination address of LP multicast is a group address ranging from 224.0.0.0 to 239.255.255.255, also called class D address. It is sort of temporary address that is dynamically allocated and restored. Every multicast group corresponds to a dynamically allocated class D address. When the multicast of a multicast group ends, the corresponding class D address is recycled for later multicast.
In order to implement multicast data packet forwarding in a real network, all interconnected devices have to run an interoperable multicast routing protocol. Multicast routing protocols are divided into: internet group management protocol (IGMP), dense mode protocol (e.g. DVMRP, PIM-DM), sparse mode protocol (e.g. PIM-SM, CBT) and link state routing protocol (MOSPF).
Wherein, the key to multicast routing is to establish a multicast tree for each multicast group, which varies from one to another according to different multicast protocols. Currently, there are two multicast tree construction techniques: source multicast tree and shared tree.
Source multicast tree is a multicast tree that is constructed from multicast sources, suitable for the cases where multicast sources are dense and the amount of multicast data is large.
Shared tree is a multicast tree that is constructed through a RP to each member of a multicast group, where multicast sources transmit the related information to the RP for multicasting. Shared tree is suitable for the cases where multicast sources are sparse and the amount of multicast data is small, so as to reduce the cost for routing information exchange of routers. Currently, the popular sparse mode multicast routing protocol is the PIM-SM (Protocol-Independent Multicast-Sparse Mode) protocol in sparse mode.